Ultrasound pulse echo imaging has become an important modality for medical diagnosis. Pulses of ultrasound energy are produced in a transducer and directed into a body. The energy is scattered from organ boundaries and other impedance discontinuities within the body; generating echos which are detected with a transducer (which may be the same transducer used for transmission) to produce electrical signals which are then processed to form an image of the internal body structures. Most ultrasound pulse echo imaging systems of the prior art generate images from information which is extracted from the AM envelope of echo signals received by the transducer. Such systems usually make use of a peak detector to extract a video signal from the returned echos and generate a display by modulating the intensity of each pixel as a function of the amplitude of a corresponding portion of video signal. Regions of the body which return strong echos (for example organ boundaries) will thus be depicted as bright areas in the image whereas regions which return low amplitude echos (for example homogenous regions within the liver) will be depicted as darker areas in the displayed image). Such apparatus is more completely described, for example, in Medical Ultrasound Imaging: An Overview of Principles and Instrumentation, J. F. Havlice and J. C. Taenzer; Proceedings of the IEEE, Volume 67, No. 4, April 1979, pages 620-640, which is incorporated herein, by reference, as background material.
More recently, Dr. Leonard Ferrari has described a technique for producing an image by utilizing information contained in the FM envelope of an ultrasound pulse echo signal. (Dr. Ferrari's U.S. patent application for "Ultrasonic Acoustic Imaging Apparatus", Ser. No. 384,533, filed on June 3, 1982 and assigned to The Regents of The University of California is incorporated herein by reference as background material.) This technique maps the instantaneous phase or frequency of the returned pulse echo signal into intensity levels in the displayed image. For example, regions of the body which return higher instantaneous frequencies may be displayed as bright areas and regions of the body which return lower frequencies may be displayed as darker areas in the image. The intensity of regions in the image will normally be independent of the amplitude of the returned signal. A squelch circuit may be provided which turns off the FM detector and displays a neutral intensity level in the event that the returned signal is too low for FM detection.
In B-scan imaging an ultrasound transducer is translated and/or angulated along the surface of a body undergoing examination. A two-dimensional image is generated by plotting the detected characteristic of a returned echo at an image point which corresponds to the coordinates of the scatterer which produced the echo. The depth coordinate of the scatterer is determined by measuring the time delay between pulse transmission and the receipt of the echo signal and the lateral coordinate of the scatterer is determined by measuring the lateral position and/or angulation of the transducer. Depth resolution in the image is primarily determined by the transfer function of the ultrasound receiver and detector. Lateral resolution in the image is determined by the lateral dimensions of the beam of ultrasound energy which is projected into the body which, in turn, is determined by the dimensions and focusing properties of the transducer.